The present invention is in the field of communications, and more particularly relates to high speed transmission of data between geographically remote data processing equipment topologically connected together so as to form a circular ring.
Generally in ring networks each piece of data processing equipment is coupled to the ring by way of Network Interface Units (NIU's) at a "node" of the network. The nodes are connected by a data transmission medium, each of the next, so as to form a single circular signal path which permits unidirectional signal flow.
Within the Network Interface Unit (NIU) of such a network it is common practice to electrically repeat (or regenerate) data as it passes through a node. Incoming data is received and demodulated so that it may be selectively inspected and/or modified by associated data processing equipment coupled to the NIU. Then the inspected data is remodulated and transmitted in order that other nodes may inspect or modify the data. By demodulating and then remodulating within each node, data may be modified as it is transmitted around the ring. As a secondary benefit, the transmitted signal is fortified and reshaped as it is repeated by each node of network allowing nodes to be physically distant.
Because of the physical separation of the nodes it is common practice to allow each node to transmit data at a rate that varies slightly from the others. This eliminates the problem of strictly enforcing a rule that each of an arbitrary number of nodes transmits at exactly the same bit rate.
This practice has several disadvantages. First, because the rate at which information is received by a node (i.e. the rate that it is transmitted by its upstream neighbor) may be slightly different from the rate that it is transmitted, data bits will necessarily accumulate in or drain out of a node. As a result, in one form of prior art system it is required that some sort of first-in/first-out (FIFO) buffer absorb the excess bits when bits are received faster than they are transmitted. Alternatively this buffer must supplement a shortage of bits when bits are transmitted faster than they are received.
As a further disadvantage, the FIFO buffer used to resolve the disparate transmit and receive rates must periodically overflow or underflow at which time data transmission must be temporarily halted. Also, the larger the FIFO, the more delay is inserted in the path followed by the data in going from the sending node to the receiving node. Conversely, short FIFO's will generally require critical control of transmit frequencies or more frequent halting of transmissions.
Alternatively, in another form of prior art system one node may be designated as a Master node, and all the remaining nodes are phase-locked to this Master node. In this way, each node except the Master, phase-locks to its upstream neighbor and thus indirectly to the Master. When this practice is employed, two disadvantages must be overcome. First, the designation of Master node needs to be periodically changed. This requires that some method be employed to select one and only one Master and that all other nodes reacquire phase-lock. Second, the Master node, when not transmitting, must be prepared to receive a signal with an arbitrary phase relationship to that that it transmits.
Both of these disadvantages stem from the requirement that on node operate differently from the rest.
It is an object of the present invention to provide an improved ring communications system.